There exist hydraulic motors, and more generally mechanisms operating with fluid under pressure, which include a cylinder capacity selector enabling them to operate with at least two different cylinder capacities, and thus to rotate at two different speeds of rotation.
In these mechanisms, pistons are disposed either radially relative to the axis of rotation or parallel to the axis of rotation, giving rise to "radial-piston" and "axial-piston" mechanisms respectively.
In these mechanisms, the various pistons are mounted to bear against undulating cams, and in axial-piston mechanisms the cams are constituted by undulating reaction plates. Further, a fluid distributor valve which rotates with the cam serves to distribute fluid to cylinders in which the pistons are slidably mounted. The various possible embodiments of such a distributor valve include a generally cylindrical shape and a shape including a plane face (known as a "plane" distributor valve). Finally, the main effect of a cylinder capacity selector is to group together the cylinders of a mechanism in a plurality of distinct cylinder groups, or to group together the feeds corresponding to respective cam undulations in a plurality of distinct cam undulation groups.
The present invention relates to such mechanisms, and particular descriptions of some such mechanisms are to be found in the following French patent specifications: FR-A No. 1 411 047 (a motor having a plurality of cylinder capacities, a cylindrical distributor valve, and a selector for grouping the cylinders in a plurality of cylinder groups); FR-A No. 1 563 866 (a motor similar to the above motor, but having a selector which groups the feeds in a plurality of cam undulation groups); and FR-A No. 2 365 041 (a motor having a plurality of cylinder capacities and including a plane distributor valve).
More particularly, the invention relates to a first type of hydraulic motor or pump mechanism constituted by:
a cylinder block;
a plurality of cylinders provided in said cylinder block, with a piston slidably mounted in each cylinder and with each cylinder being provided with an orifice for communication with the outside thereof;
a cam, said cylinder block being rotatably mounted relative to said cam about an axis of rotation, said pistons being suitable for bearing against the surface of said cam, and said cam comprising a plurality of slopes which follow one another in pairs, with each pair including a first slope going away from the cylinder block relative to the general direction of piston sliding while pressed against said cam, and with the second slope of each pair sloping towards the cylinder block relative to the general direction of piston sliding;
two enclosures, one suitable for containing high pressure fluid and the other for containing low pressure fluid;
a fluid distributor valve constrained to rotate with the cam, and including as many pairs of orifices as the cam includes pairs of slopes, with the two orifices of a pair of orifices comprising a first orifice which corresponds to the first slope of a pair of slopes and a second orifice which corresponds to the second slope of the same pair of slopes, and with the orifice for putting each cylinder into communication with the outside of the cylinder being successively brought into communication with each pair of orifices in the distributor during rotation of the cylinder block relative to the cam; and
a selector for selecting the active cylinder capacity of the mechanism and capable of conferring at least two distinct configurations on said mechanism, including a first configuration in which communication is established firstly between all of said first orifices of said pairs of distributor valve orifices and a first one of said two enclosures, and secondly between all said second orifices of said pairs of distributor valve orifices and the second of said two enclosures, and a second configuration in which the pairs of distributor valve orifices are grouped in at least two groups of pairs of orifices corresponding to at least two groups of pairs of cam slopes, said selector firstly establishing said first configuration communication solely for the orifices of a first of said two groups of pairs of distributor valve orifices, and secondly isolating at least one of said two enclosures from at least one of said first and second orifices of the pairs of orifices of the second of said two groups of pairs of orifices.
The invention also relates to a second type of hydraulic motor or pump mechanism constituted by:
a cylinder block;
a plurality of cylinders provided in said cylinder block, with a piston slidably mounted in each cylinder and with each cylinder being provided with an orifice for communication with the outside thereof;
a cam, said cylinder block being rotatably mounted relative to said cam about an axis of rotation, said pistons being suitable for bearing against the surface of said cam, and said cam comprising a plurality of slopes which follow one another in pairs, with each pair including a first slope going away from the cylinder block relative to the general direction of the piston sliding while pressed against said cam, and with the second slope of each pair sloping towards the cylinder block relative to the general direction of the piston sliding;
two enclosures, one suitable for containing high pressure fluid and the other for containing low pressure fluid;
a fluid distributor valve constrained to rotate with the cam, and including as many pairs of orifices as the cam includes pairs of slopes, with the two orifices of a pair of orifices comprising a first orifice which corresponds to the first slope of a pair of slopes and a second orifice which corresponds to the second slope of the same pair of slopes, and with the orifice for putting each cylinder into communication with the outside of the cylinder being successively brought into communication with each pair of orifices in the distributor during rotation of the cylinder block relative to the cam; and
a selector for selecting the active cylinder capacity of the mechanism and suitable for conferring at least two different configurations to said mechanism, including a first configuration in which the selector establishes communication during said relative rotation between the orifice for putting each cylinder into communication with the outside from said cylinder successively with each of the two orifices in each pair of distributor valve orifices, and a second configuration in which the cylinders are distributed together with their said communication orifices in at least two groups of cylinders and two corresponding groups of communication orifices, with the selector establishing first configuration communication solely for the communication orifices of a first group of said two groups of cylinder communication orifices while simultaneously isolating at least one of said two enclosures from the communication orifices of the second group of cylinder communication orifices, said cylinders being distributed around the axis of relative rotation between the cylinder block and the cam with the axes of any pair of consecutive cylinders being at substantially at the same angular intervals.
In the above known types of hydraulic mechanism, two complementary dispositions are adopted. The first complementary disposition consists in selecting cam profiles so that each of the operating cylinder capacities remains constant: such a mechanism is known as a "constant-velocity" mechanism and this disposition has the advantage of obtaining a constant speed of rotation for the mechanism at a given cylinder capacity and a given fluid flow rate. The second complementary disposition consists in providing the smaller operating cylinder capacity by preventing some of the cylinders from communicating with the high and low pressure enclosures of the mechanism, and by choosing these cylinders so that they are regularly distributed angularly around the axis of rotation. For example, during one turn about the axis of rotation every other cylinder may be isolated in succession from at least one of the said two enclosures. This method of proceeding has an obvious advantage which is unfortunately accompanied by a very awkward drawback. The advantage obtained is to maintain a zero or substantially zero resultant of the forces due to the effects of the fluid under pressure contained in those cylinders which are still active. The serious drawback observed lies in the fact that in order to maintain constant each of the possible operating cylinder capacities, calculation shows that the cam profiles must have small radiuses of curvature, and in particular radiuses which are so small that the corresponding high contact pressures exerted on the cam significantly reduce the lifetime of the mechanism. Thus, known multiple cylinder capacity mechanisms are indeed contant-velocity mechanisms and they do indeed have force resultants perpendicular to the axis of rotation which are small or zero, but their lifetimes are shortened by premature destruction of the cams and/or the members which bear against them.
The invention consists in overcoming the technical prejudice relating to obtaining a small or zero resultant for forces perpendicular to the axis of rotation, i.e. for "radial forces" as described above, in selecting a solution which, a priori is bad from this point of view, i.e. in selecting a solution which leads to a non-zero resultant of these forces being generated and which thus leads to the corresponding mechanism being inherently somewhat out-of-balance by its very design; and then in showing that it is possible to make constant velocity mechanisms having a plurality of (i.e. at least two) different operating cylinder capacities with cam profiles which have satisfactory radiuses of curvature. The above-mentioned severe drawback of prior mechanisms is thus avoided and this type of mechanism is rendered usable in an industrial context, albeit at the price of generating a parasitic resultant force on the axis of rotation. After daring to propose a solution which is a priori bad, and thus genuinely non-obvious to the person skilled in the art, the applicant's invention has further merit which is just as remarkable: not only is it possible to eliminate the drawback of the prior art (relating to the bad cam profile), but it is also possible to find a solution to the new drawback (that of generating a non-zero resultant axial force): rotary bearings currently available on the market are quite capable of absorbing the above-mentioned resultant force without difficulty.